Pneumatic radial tire for motorcycle

ABSTRACT

There is provided a pneumatic radial tire for a motorcycle capable of developing an excellent steering stability in a high-speed cornering subjected to a large camber angle, and having an aspect ratio of 0.50-0.85, in which a tensile strength at break of a carcass ply cord is not less than  980  MPa, and an absolute value of a carcass total rigidity obtained by adding carcass rigidities of carcass plies is not less than 30000, and a tensile strength at break of a belt layer cord is not less than  2350  MPa, and an absolute value of a belt total rigidity obtained by adding belt rigidities of belt layers is not less than 170000, and an out-of-plane bending rigidity in widthwise direction of a tread portion is 0.40-0.70 kg/mm and an in-plane bending rigidity in peripheral direction thereof is 0.05-0.15 kg/mm and a bending rigidity ratio as a ratio of out-of-plane bending rigidity to in-plane bending rigidity is within a range of 4.20-9.10.

TECHNICAL FIELD

This invention relates to a pneumatic radial tire suitable for amotorcycle conducting a high-speed cornering while contacting a sideedge portion of a tire tread with ground, particularly a front wheelthereof, and more particularly it proposes a technique that a steeringstability in the high-speed cornering is largely improved whilesufficiently developing various performances such as a high-speeddurability, a straight-running stability and the like.

BACKGROUND ART

In the cornering of the motorcycle on urban areas, when a certain camberangle is given to the tire, it is generally and widely conducted tocontact a tread contacting face with ground under a condition that aposition of surface width measured from an equatorial plane of the tireis limited to about 50-75% of a tread half-width.

Therefore, the pneumatic radial tire for the motorcycle exclusively usedin the running on urban areas is possible to sufficiently develop thedesired performances by giving a large camber angle without deeplypursuing optimization on various bending rigidities of a belt, arigidity balance and the like, which highly exerts on the steeringstability in the high-speed cornering while contacting the treadcontacting face with ground up to a side edge position.

In recent years, however, the motorcycle is made to higher performances,and also special control areas for sporty running such as trainingfield, circuit and the like, at where amateur riders can enjoy drasticsporty running while challenging to a limit of the steering technique,other than public roads are increasing. As the sporty running is readilyenjoyed, the cornering at a high speed while largely inclining a vehiclebody, or so-called large camber running is frequently conducted. In thetires based on the conventional belt design technique and the like,therefore, it is increasing to be a dissatisfaction that the steeringstability in the high-speed cornering is lacking in such a large camberrunning.

The invention is to solve the above problems inherent to theconventional pneumatic motorcycle tire and to provide a pneumatic radialtire for a motorcycle capable of sufficiently developing variousperformances such as high-speed durability, straight-running stabilityand the like but also developing an excellent steering stability in theaforementioned large camber running.

DISCLOSURE OF THE INVENTION

The pneumatic radial tire for a motorcycle according to the invention isa tire comprising a pair of bead portions, a pair of sidewall portions,a tread portion toroidally extending and continuing to the sidewallportions, a carcass comprised of one or more carcass plies containingorganic fiber cords each extended at an angle of 60-90° with respect toan equatorial plane of the tire, and a belt comprised of at least twobelt layers each containing cords extended at an angle of 15-40° withrespect to the equatorial plane and disposed at an outer peripheral sideof a crown portion of the carcass so as to cross the cords of the layerswith each other, and having an aspect ratio of 0.50-0.85, in which

-   -   a tensile strength at break (Edci) of the carcass ply cord is        not less than 980 MPa and an absolute value of a carcass total        rigidity (Fc=ΣFci) obtained by adding a carcass rigidity (Fci)        defined as a product of the tensile strength at break (Edci) and        an end count (Emci) of the carcass ply cords per a length of 50        cm in the equatorial plane of the tire with respect to the        respective carcass plies is not less than 30000, and    -   a tensile strength at break (Edbj) of the belt layer cord is not        less than 2350 MPa and an absolute value of a belt total        rigidity (Fb=ΣFbj) obtained by adding a belt rigidity (Fbj)        defined as a product of the tensile strength at break (Edbj) and        an end count (Nmbj) of the belt layer cords per a length of 50        cm in the equatorial plane of the tire with respect to the        respective belt layers is not less than 170000, and    -   a bending rigidity ratio (Sb/Sa) of out-of-plane bending        rigidity in a widthwise direction (Sb) to in-plane bending        rigidity in a peripheral direction (Sa) in the tread portion is        within a range of 4.20-9.10.

In the tread portion of such a tire, it is more preferable that theout-of-plane bending rigidity in the widthwise direction (Sb) is3.92-6.86 N/mm and the in-plane bending rigidity in the peripheraldirection (Sa) is 0.49-1.47 N/mm.

The term “aspect ratio” used herein is an “aspect ratio” defined inJATMA YEAR BOOK or a “NOMINAL ASPECT RATIO” defined in ETORTO STANDARDMANUAL.

Also, the in-plane bending rigidity in the peripheral direction of thetread portion (Sa) means a rigidity to a force acting to a peripheralcomponent in the tread portion from a widthwise direction of the tread.Concretely, it can be determined by measuring and calculating(force/displacement) when a sample A of 15 mm in width formed by cuttinga tread portion of a product tire in the peripheral direction of thetire as shown by a solid line in FIG. 1 is set to a measuring device asshown in FIG. 2 at a posture of directing cut faces up and down and apushing force is applied to an up-side cut face.

On the other hand, the out-of-plane bending rigidity in the widthwisedirection of the tread portion (Sb) means a rigidity to an externalforce acting to a widthwise component of the tread portion in a pushingdirection of the tread contacting face. This rigidity can be determinedby measuring and calculating (force/displacement) when a sample B of 15mm in width formed by cutting a tread portion of a product tire in thewidthwise direction of the tread as shown by a phantom line in FIG. 1 isset to a measuring device as shown in FIG. 2 at a posture of renderingthe tread contacting face into an up-side face and a pushing force isapplied to the up-side face.

The terms “rigidity” and “tensile strength at break” used herein meanvalues obtained by a given measurement at normal temperature of 25° C.,respectively.

In the pneumatic radial tire for the motorcycle having the aboveconstruction, the tensile strength at break (Edci) of the carcass plycord is made to not less than 980 MPa and the tensile strength at break(Edbj) of the belt layer cord is made to not less than 2350 MPa, wherebya desired tire case strength can be ensured without increasing thenumber of carcass plies, end count of the carcass ply cords and thelike, and a belt strength enough to provide an excellent hoop effect inthe high-speed running can be ensured without increasing the number ofthe belt layers and the like. In this case, therefore, the increase oftire weight can be prevented.

Also, the absolute value of the carcass total rigidity (Fc) is made tonot less than 30000 and the absolute value of the belt total rigidity ismade to not less than 170000, whereby a desired durability can be givento the tire. In other words, when the absolute value of the carcasstotal rigidity is less than 30000, it is difficult to ensure thestraight-running stability and the steering stability in the high-speedcornering, while when the absolute value of the belt total rigidity isless than 170000, it is difficult to ensure the high-speed durabilityand the steering stability in the high-speed cornering.

Moreover, it is preferable that a rigidity ratio (|Fc/Fb|) as a ratio ofabsolute value of carcass total rigidity (Fc) to absolute value of belttotal rigidity (Fb) is within a range of 0.10-0.50.

That is, when the belt rigidity is too high, there is a fear that thesteering stability in the high-speed cornering lowers due to therigidity difference to the carcass though the high-speed durability isimproved, while when the belt rigidity is too low or when the carcassrigidity is too high, the high-speed durability lowers or the rigidityof a tire side portion becomes high to make kicking-back strong andthere is a fear of lowering the straight-running stability, and also apeaky change of the cornering performance is liable to be easily caused.Therefore, it is preferable to ensure an excellent rigidity balance byselecting the rigidity ratio within the range of 0.10-0.50.

Particularly, in the running of the motorcycle, the ground contactingposture of the front wheeled tire is kaleidoscopically changed by thebraking in the going into a corner, application of camber angle andhandling in the cornering, straight-running after the passing throughthe corner and the like, or various inputs are repeatedly applied to thetire. In such a running, when operations such as application of arelatively small camber angle to the tire based on a little banking of avehicle body, handling and braking are conducted together in thevicinity of an entrance of the corner, it is preferable that thein-plane bending rigidity of the tread portion in the peripheraldirection (Sa) is made to not less than 0.49 N/mm for particularlyensuring the steering stability in the high-speed cornering.

In the cornering on the corner by mixing the violent banking of thevehicle body and the handling, in order to enhance the road holdingproperty by flexibly deforming the tread crown portion based on areaction force of a road surface, it is preferable that the out-of-planebending rigidity of the tread portion in the widthwise direction (Sb) isrestricted to not more than 6.86 N/mm. On the other hand, when theout-of-plane bending rigidity (Sb) is less than 3.92 N/mm, the rigiditycomes short of.

Under the above viewpoint, when the value of the in-plane bendingrigidity of the tread portion in the peripheral direction (Sa) exceeds1.47 N/mm, the flexible contacting of the tread crown portion withground is obstructed if the out-of-plane bending rigidity (Sb) is withinthe above selected range.

In order to balancedly ensure the adequate rigidity of the tread crownportion and the adequate flexibility, it is required that the abovebending rigidities of the tread portion are set in the proper range butalso the ratio of these bending rigidities (Sb/Sa) is set in a range of4.20-9.10.

That is, when the ratio is less than 4.20, it is difficult to ensure thestraight-running property, while when it exceeds 9.10, it is difficultto ensure the steering stability in the high-speed running.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a method of cutting out a sample for themeasurement of bending rigidity.

FIG. 2 is a schematic view illustrating an apparatus for the measurementof bending rigidity.

FIG. 3 is a section view in a widthwise direction of a tread accordingto an embodiment of the invention.

FIG. 4 is a graph showing a change of steering stability to anout-of-plane bending rigidity in a widthwise direction of a treadportion.

FIG. 5 is a graph showing a change of steering stability to an in-planebending rigidity in a peripheral direction of a tread portion.

FIG. 6 is a graph showing a change of steering stability to a bendingrigidity ratio.

FIG. 7 is a graph showing a change of steering stability to a rigidityratio between carcass and belt.

BEST MODE FOR CARRYING OUT THE INVENTION

In an embodiment of the invention shown in FIG. 3, numeral 1 is a pairof bead portions, numeral 2 a sidewall portion continuing to each of thebead portions 1 and extending outward therefrom in a radial direction,and numeral 3 a tread portion toroidally extending and continuing toeach of the sidewall portions. The illustrated tire has an aspect ratioof 0.50-0.85.

A radial carcass 5 reinforcing the above portions 1, 2, 3 is arranged soas to toroidally extend between bead cores 4 arranged in the respectivebead portions 1. The carcass 5 is comprised of at least one carcass ply6 containing organic fiber cords each extended at an angle of 60-90°with respect to an equatorial plane S of the tire.

On an outer peripheral side of a crown portion of the radial carcass 5is arranged a belt 9 comprised of two or more belt layers 7, 8 in whichbelt layer cords are crossed with each other between these layers,preferably these cords extend in a direction opposite to each other withrespect to the equatorial plane of the tire and an angle of the beltlayer cord with respect to the equatorial plane S of the tire is withina range of 15-40°.

The organic fiber cord constituting the carcass ply 6, i.e. the carcassply cord has a tensile strength at break (Edci) of not less than 980MPa, while an absolute value of a carcass total rigidity obtained byadding a carcass rigidity (Fci) defined as a product of the tensilestrength at break (Edci) and an end count (Emci) of the carcass plycords per a length of 50 cm in the equatorial plane of the tire withrespect to the respective carcass plies 6 is set in not less than 30000.

In addition, the cord of each belt layer has a tensile strength at break(Edbj) of not less than 2350 MPa, while an absolute value of a belttotal rigidity (Fb) obtained by adding a belt rigidity (Fbj) defined asa product of the tensile strength at break (Edbj) and an end count(Nmbj) of the belt layer cords per a length of 50 cm in the equatorialplane of the tire with respect to the respective belt layers 7, 8 is setin not less than 170000.

Moreover, the rigidity ratio (|Fc/Fb|) as a ratio of absolute value ofcarcass total rigidity to absolute value of belt total rigidity is setin a range of 0.10-0.50.

In the tread portion 3, as previously mentioned in relation to FIGS. 1and 2, the in-plane bending rigidity in the peripheral direction (Sa) isset in a range of 0.49-1.47 N/mm, and the out-of-plane bending rigidityin the widthwise direction (Sb) is set in a range of 3.92-6.86 N/mm,while the rigidity ratio (Sb/Sa) as a ratio of out-of-plane bendingrigidity (Sb) to in-plane bending rigidity (Sa) is set in a range of4.20-9.10.

According to the pneumatic tire for the motorcycle having the aboveconstruction, the development of high steering stability in the largecamber running can be guaranteed by selecting the tensile strength atbreak of the respective cord, various rigidities and rigidity ratiowithin the above ranges without increasing the tire weight and with thesufficient securement of basic performances such as excellent high-speeddurability, straight-running property and the like.

Moreover, it is preferable in such a tire that when the tire isassembled onto a rim defined by a standard of JATMA YEAR BOOK, ETORTOSTANDARDS MANUAL or the like and an air pressure defined by the samestandard is filled in the tire, a tread curvature under no load as aratio of a distance h in radial direction from a tire outermost point Pon the equatorial plane S to a position of a tread maximum width to atread maximum width TW equal to a tire maximum width in the figure isset to not less than 0.23 but not more than 0.5.

That is, when the curvature is less than 0.23, the ground contactingproperty in the cornering lowers and it is difficult to ensure thestability in the cornering, while when it exceeds 0.50, there is a fearthat it is difficult to generate sufficient lateral force.

EXAMPLE 1

An actual running test is carried out by a motorcycle using a frontwheel tire having a tire size of 120/70 ZR17 (rim width: 3.50 inch,internal pressure: 206 kPa) and a rear wheel tire having a tire size of190/55 ZR17 (rim width: 6.00 inch, internal pressure: 186 kPa). In thistest, when an out-of-plane bending rigidity in the widthwise directionis variously changed in the tread portion of the front wheel tire, thesteering stability in the high-speed cornering is evaluated by adriver's feeling to obtain results as shown by a graph in FIG. 4.

In this case, the cornering speed is 120 km/h, and as the index of thesteering stability becomes larger, the result is better.

In the graph, a comparative tire of the following Table 1 is a controltire and the performance thereof is 100 as an index.

According to FIG. 4, when the out-of-plane bending rigidity (Sb) exceeds6.86 N/mm, the steering stability lowers to the same extent as thecontrol tire resulted from the lowering of the road holding property ofthe tire. While, when the out-of-plane bending rigidity is less than3.92 N/mm, the lowering of the steering stability become conspicuous dueto the lacking of the rigidity.

EXAMPLE 2

A change of the steering stability when the in-plane bending rigidity inthe radial direction (Sa) is variously changed in the tread portion ofthe front wheel tire is measured in the same manner as in Example 1 toobtain results as shown by a graph in FIG. 5.

In this case, the comparative tire of Table 1 is also control, and theperformance thereof is 100 as an index.

According to FIG. 5, when the in-plane bending rigidity (Sa) is not lessthan 0.49 N/mm, an excellent steering stability in the cornering can bedeveloped as compared with the control tire, while when it exceeds 1.47N/mm, the soft contact of the tread crown portion with ground isobstructed and it is obliged to cause the violent lowering of thesteering stability.

EXAMPLE 3

A change of the steering stability by changing the bending rigidityratio (Sb/Sa) in the tread portion of the front wheel tire is measuredin the same manner as in Example 1 to obtain results as shown in FIG. 6.

In this case, the control tire is also the comparative tire of Table 1,and the performance thereof is 100 as an index.

As seen from FIG. 6, when the bending rigidity ratio (Sb/Sa) exceeds9.10, the steering stability violently lowers to an extent lower thanthat of the control tire, and the similar tendency is caused even whenit is less than 4.20.

EXAMPLE 4

A change of the steering stability by changing the rigidity ratio(|Fc/Fb|) as a ratio of absolute value of carcass total rigidity toabsolute value of belt total rigidity (Fb) is measured in the samemanner as in Example 1 to obtain results as shown in FIG. 7.

In this case, the control tire is the comparative tire of Table 1.

As seen from FIG. 7, when the rigidity ratio (|Fc/Fb|) is not less than0.10, the steering stability is largely improved, while when it exceeds0.50, the steering stability lowers to the same extent as in the controltire.

EXAMPLE 5

An actual running test is carried out by a motorcycle using a frontwheel tire having a tire size of 120/70 ZR17 (rim width: 3.50 inch,internal pressure: 206 kPa) and a rear wheel tire having a tire size of190/55 ZR17 (rim width: 6.00 inch, internal pressure: 186 kPa), in whichthe steering stability in the high-speed turning is evaluated by adriver's feeling to obtain results shown in Table 1.

In Table 1 are also shown constitutional forms of an example tire and acomparative tire used as a front wheel tire, respectively. TABLE 1 Frontwheel tire Comparative Example tire Aspect ratio 0.70 0.70 Carcass oneply one ply Cord nylon nylon Tensile strength at break Edci 980 MPa 980MPa Angle (to peripheral direction of tire) 90° 90° Carcass totalrigidity Fc (absolute value) 70000 70000 Belt three layers two layersCord aramid aramid Tensile strength at break Edbj 2800 MPa 2800 MPaAngle (to peripheral direction of tire) 30° 23° Belt total rigidity Fb(absolute value) 300000 200000 In-plane bending rigidity in peripheral1.078 N/mm 2.156 N/mm direction Sa Out-of-plane bending rigidity in 5.39N/mm 9.212 N/mm widthwise direction Sb Tread curvature Rc 0.23 0.22Steering stability in high-speed cornering 115 100 Feeling evaluation(the larger the index value, the better)

As seen from Table 1, the example tire can largely improve the steeringstability in the high-speed cornering because various rigidityrequirements and the like are satisfied.

INDUSTRIAL APPLICABILITY

As seen from the above, according to the invention, the high steeringstability can be developed even when the tire is subjected to a largecamber angle enough to contact the side edge portion of the treadcontacting area with ground.

1. A tire comprising a pair of bead portions, a pair of sidewallportions, a tread portion toroidally extending and continuing to thesidewall portions, a carcass comprised of one or more carcass pliescontaining organic fiber cords each extended at an angle of 60-90° withrespect to an equatorial plane of the tire, and a belt comprised of atleast two belt layers each containing cords extended at an angle of15-40° with respect to the equatorial plane and disposed at an outerperipheral side of a crown portion of the carcass so as to cross thecords of the layers with each other, and having an aspect ratio of0.50-0.85, in which a tensile strength at break (Edci) of the carcassply cord is not less than 980 MPa and an absolute value of a carcasstotal rigidity (Fc=ΣFci) obtained by adding a carcass rigidity (Fci)defined as a product of the tensile strength at break (Edci) and an endcount (Emci) of the carcass ply cords per a length of 50 cm in theequatorial plane of the tire with respect to the respective carcassplies is not less than 30000, and a tensile strength at break (Edbj) ofthe belt layer cord is not less than 2350 MPa and an absolute value of abelt total rigidity (Fb=ΣFbj) obtained by adding a belt rigidity (Fbj)defined as a product of the tensile strength at break (Edbj) and an endcount (Nmbj) of the belt layer cords per a length of 50 cm in theequatorial plane of the tire with respect to the respective belt layersis not less than 170000, and a bending rigidity ratio (Sb/Sa) ofout-of-plane bending rigidity in a widthwise direction (Sb) to in-planebending rigidity in a peripheral direction (Sa) in the tread portion iswithin a range of 4.20-9.10.
 2. A pneumatic radial tire for a motorcycleaccording to claim 1, wherein the out-of-plane bending rigidity in thewidthwise direction (Sb) is 3.92-6.86 N/mm and the in-plane bendingrigidity in the peripheral direction (Sa) is 0.49-1.47 N/mm.
 3. Apneumatic radial tire for a motorcycle according to claim 1, wherein arigidity ratio (|Fc/Fb|) as a ratio of absolute value of carcass totalrigidity (Fc) to absolute value of belt total rigidity (Fb) is within arange of 0.10-0.50.
 4. A pneumatic radial tire for a motorcycleaccording to claim 2, wherein a rigidity ratio (|Fc/Fb|) as a ratio ofabsolute value of carcass total rigidity (Fc) to absolute value of belttotal rigidity (Fb) is within a range of 0.10-0.50.